Are there any freshwater cephalopods?

Do freshwater cephalopods exist, and if not, what is the most likely reason why not?

Earlier this month, scientists released the first images of a giant squid swimming in deep water off the coast of Japan. The elusive cephalopod, which can grow up to 14 metres long, is one of the biggest invertebrates in the ocean.
(Source: AFP)

Most aquatic species have fresh water and salt water varieties, however I cannot remember hearing of a freshwater cephalopod. Do they exist, and if not, what is the most likely reason why not?

Cephalopods include squid, cuttlefish, octopuses and nautiluses. All 1000 or so species are voracious carnivores, with large brains, complex behaviours and the ability to change colour, texture and shape to elude hungry predators keen to snack on the soft-bodied animals.

They are found throughout the world's oceans from the warm waters of the tropics down to subzero temperatures in polar regions. They can survive out of water for extended periods of time, and some, like the vampire squid Vampyroteuthis infernalis escape from predators by hiding in water so low in oxygen that the fish chasing them would pass out.

Another new species of pale octopus was found recently living at 2400 metres depth around hydrothermal vents near Antarctica. These vents produce high concentrations of hydrogen sulphide and temperatures exceed 382 degrees Celsius.

But while cephalopods are clearly adept at exploiting extreme environments, they are not found in freshwater.

"While we can't be 100 per cent certain, it's unlikely that there have ever been freshwater cephalopods," says cephalopod expert and Head of Science at Museum Victoria, Dr Mark Norman.

It's all to do with osmosis.

"It is probable that they never developed a sodium pump that would help them cope with osmotic change in freshwater," Norman explains.

Freshwater dwellers have salty blood relative to the water around them. Without a mechanism in place to control it, osmosis would equalise salt concentrations between the animal and the water surrounding it, pumping salt out of the body and flooding it with freshwater. A sodium pump, like that found in freshwater fish species, uses chloride cells on the gill surface to actively absorb sodium and potassium ions from the environment. Any excess water taken in at the same time is excreted as urine.

Marine dwellers have the opposite problem, and need to conserve fresh water while expelling salt. Cephalopods pump seawater through their gills and use their kidneys to filter out fresh water from the ocean. Salts and waste water are channelled through the funnel.

Need for speed

Even if they did initially evolve a mechanism for coping with freshwater, Norman says that as they needed to outswim increasingly fast fish predators, and to become supercharged top-order predators themselves, many functions were lost. This allowed others to develop to extremes.

"Cephalopods have transparent blue, runny, copper-based blood that doesn't carry a lot of oxygen," he says.

"In order to maximise the amount of oxygen that could be carried by the hemocyanin in their blood, and to get their blood pressure high enough to pump blood to high speed muscles, they evolved three hearts," he adds. "In our bodies our hearts pump blood in one loop around the brain and in another around the body.

"Cephalopods have a heart above each gill (known as branchial hearts) as well as a central (systemic) heart that pumps blood around the body."

The structure of their complex nervous system also maximises speed and efficiency - either for prey capture or for escape from danger.

"They have complex visual senses, camouflage and signalling skills.

"The brains of cephalopods are ring shaped, and encircle the oesophagus, requiring them to pulversie their food before swallowing it," Norman explains.

Some nerve cells also control chromatophores, skin colour cells that, similar to a picture on a high definition television, can change instantly for camouflage. Other nerve cells also help them disappear in the blink of an eye when threatened.

"The squid giant axon [a giant nerve cell, not to be confused with a nerve cell from a giant squid] is the largest of any animal and controls escape behaviours," he says, adding that a single nerve cell can be 30 centimetres long, and up to 1 millimetre in diameter (the diameter of a human nerve cell is 0.1 millimetres by comparison).

"This means that the second the second the brain says 'go!' they're off. There is no delay as the message is transmitted between synapses as it is in other animals."

Dr Mark Norman is the Head of Science at Museum Victoria, Dr Norman spoke to Rachel Sullivan.